Embedded permeable polysilicon layer in MEMS device for multiple cavity pressure control

Abstract

Disclosed herein is a process flow for forming a MEMS IMU including an accelerometer and a gyroscope each located in a separate sealed cavity maintained at a different pressure. Formation of the MEMS IMU includes the use of a first vHF release to etch a sacrificial layer underneath a structural layer containing the accelerometer and gyroscope and capping the device under formation to set both cavities at a first pressure. The floor of one of the cavities is formed to including a gas permeable layer. Formation further includes forming a chimney underneath the gas permeable layer and then performing a second vHF release to etch through the gas permeable layer and expose the cavity containing the gas permeable layer so that its pressure may be set to be different than that of the other cavity when the chimney is sealed.

The invention claimed is: 
     
       1. A method of forming a package including a first instrument positioned within a first cavity maintained at a first pressure and a second instrument positioned within a second cavity maintained at a second pressure different than the first pressure, the method comprising:
 forming an oxide layer on a substrate; 
 depositing a permeable layer on the oxide layer, the permeable layer being gas permeable but vapor etch resistant; 
 depositing an insulator layer on the permeable layer and exposed portions of the oxide layer; 
 depositing a barrier layer on the insulator layer; 
 depositing a sacrificial layer over exposed portions of the insulator layer; 
 etching the sacrificial layer and insulator layer at portions thereof overlying the permeable layer so as to expose portions of the permeable layer; 
 forming a structural layer over the sacrificial layer and exposed portions of the permeable layer; 
 forming the first and second instruments in the structural layer; 
 performing a first vapor etching through open portions of the structural layer to remove exposed portions of the sacrificial layer, the first vapor etching also passing through the permeable layer at the exposed portions thereof to thereby remove portions of the oxide layer underlying the permeable layer to expose portions of the substrate; 
 bonding a cap wafer to the structural layer at bonding points to thereby:
 form the first cavity as being sealed by the cap wafer, portions of the barrier layer underlying the structural layer adjacent the first instrument, and the bonding points; and 
 form the second cavity as being sealed by the cap wafer, portions of the barrier layer, underlying the structural layer adjacent the second instrument, and the exposed portions of the substrate; 
 
 wherein the bonding of the cap wafer to the structural layer is performed at a first pressure, thereby setting pressure within the first and second cavities to the first pressure; 
 forming a chimney opening extending through an entire thickness of the substrate to reach the permeable layer; 
 performing a second vapor etching through the chimney opening and permeable layer to remove portions of the oxide layer under the permeable layer and portions of the insulator layer stacked on the permeable layer, thereby creating a fluid connection extending from the chimney opening into the second cavity; 
 setting the pressure inside the second cavity to a second pressure different than the first pressure; and 
 forming a plug in the chimney opening to thereby seal the second cavity. 
 
     
     
       2. The method of  claim 1 , wherein the oxide layer is a thermal oxidation layer. 
     
     
       3. The method of  claim 1 , wherein the permeable layer is comprised of a permeable polysilicon material. 
     
     
       4. The method of  claim 1 , wherein the insulator layer is comprised of silicon dioxide SiO2. 
     
     
       5. The method of  claim 1 , wherein the barrier layer is comprised of aluminum oxide or silicon nitride. 
     
     
       6. The method of  claim 1 , wherein the sacrificial layer is comprised of silicon dioxide SiO2. 
     
     
       7. The method of  claim 1 , wherein the structural layer is formed from polysilicon. 
     
     
       8. The method of  claim 1 , wherein the first instrument is an accelerometer and the second instrument is a gyroscope; and wherein the second pressure is less than the first pressure. 
     
     
       9. The method of  claim 1 , wherein the bonding of the cap wafer to the structural layer is performed by glass frit bonding, eutectic AlGe, or Au bonding. 
     
     
       10. The method of  claim 1 , further comprising prior to depositing the sacrificial layer, forming a runner layer on the barrier layer to create an interconnection layer; wherein the sacrificial layer is also deposited over the runner layer; wherein the runner layer is also etched along with the sacrificial layer and insulator layer to expose the portions of the permeable layer; wherein the structural layer is also formed over exposed portions of the runner layer; wherein the first cavity is also sealed by portions of the runner layer; wherein the second cavity is also sealed by portions of the runner layer; and wherein the second vapor etching is also performed to remove portions of the runner layer stacked on the permeable layer. 
     
     
       11. The method of  claim 10 , further comprising exposing a portion of the interconnection layer. 
     
     
       12. A method of forming a package including a first instrument positioned within a first cavity maintained at a first pressure and a second instrument positioned within a second cavity maintained at a second pressure different than the first pressure, the method comprising:
 performing a first vapor etching to etch a sacrificial layer underneath a structural layer containing the first and second instruments to thereby delimit a bottom of the first and second cavities; 
 bonding a cap wafer over the first and second cavities to thereby delimit a top of the first second cavities, with a pressure during the bonding setting the first and second cavities to be at the first pressure; 
 forming a chimney underneath a permeable layer disposed on a bottom of the second cavity; 
 performing a second vapor etching through the chimney and the permeable layer to expose the second cavity; and 
 forming a plug in the chimney to thereby re-seal the second cavity, with a pressure during the formation of the plug setting the second cavity to be at the second pressure, the second pressure being less than the first pressure. 
 
     
     
       13. The method of  claim 12 , wherein the permeable layer is comprised of a permeable polysilicon material. 
     
     
       14. The method of  claim 12 , wherein the first instrument is an accelerometer and the second instrument is a gyroscope; and wherein the second pressure is less than the first pressure. 
     
     
       15. The method of  claim 12 , wherein the bonding of the cap wafer is performed by bonding the cap wafer to a structural layer containing the first and second instruments. 
     
     
       16. The method of  claim 15 , wherein the bonding is performed by glass frit bonding, eutectic AlGe, or Au bonding. 
     
     
       17. The method of  claim 15 , wherein the structural layer is formed from polysilicon. 
     
     
       18. A package, comprising:
 a substrate having a through-hole defined in its back face; 
 an oxide layer disposed on a front face of the substrate, with an opening defined in the oxide layer over the through-hole in the substrate; 
 a permeable layer disposed on the oxide layer and extending across the opening in the oxide layer; 
 an insulator layer disposed at least on the oxide layer, with an opening defined in the insulator layer at least partially over the permeable layer; 
 a structural layer positioned over the insulator layer and permeable layer, the structural layer having first and second instruments formed therein; and 
 a cap layer bonded to the structural layer; 
 wherein the insulator layer, structural layer, and cap layer are cooperative shaped so as to define a first sealed cavity containing the first instrument and a second sealed cavity containing the second instrument and the permeable layer; and 
 wherein a first air pressure within the first sealed cavity is different than a second air pressure within the second sealed cavity. 
 
     
     
       19. The package of  claim 18 , further comprising a sealing plug positioned within the through-hole in the substrate, the sealing plug being shaped so as to environmentally seal a back face of the permeable layer against environmental instrusion. 
     
     
       20. The package of  claim 18 , wherein the first instrument is an accelerometer and the second instrument is a gyroscope; and wherein the second air pressure is less than the first air pressure.